I. Field of the Invention
The invention relates to position sensors, in particular their detector unit, based on the runtime measurement principle for mechano-elastic density waves (MEDW) in a wave conductor which include a position element in addition to the wave conductor, wherein the position element is moveable relative to the wave conductor and generates or detects the MEDW.
II. Prior Art
The wave conductor is typically made from a tube, a wire or a band and can also be used as an electrical conductor. The wave conductor can furthermore be arranged in a shaping linear or circular element made from non-magnetic material, e.g. plastic or metal for receiving and storing the wave conductor.
Based on the Wiedemann effect, a power impulse fed into the wave conductor when superimposed with an external magnetic field laterally oriented relative to the magnetostrictive wave conductor, wherein the external magnetic field is caused by the position element, in particular a position magnet, causes a torsion impulse of a MEDW which propagates at a speed of approximately 2,500 m/s to 6,000 m/s from its origin thus e.g. the position of the position elements in both directions in the wave conductor as a function of the elasticity modulus or of the shear elasticity moduli of the wave conductor material employed.
At one location, typically at one end of the wave conductor in particular the torsion component of the mechano-elastic density impulse is detected by a detector unit which is typically in a fixed position with reference to the wave conductor. The time period between triggering the excitation power impulse and receiving the MEDW thus is a measure for the distance of the movable position element, e.g. of the position magnet from the detector device or also the coil or the electric magnet.
A typical sensor of this type is described in U.S. Pat. Nos. 5,590,091 and 5,736,855.
The main object of the present invention is the detector device.
A prior art detector device includes a detector coil which is either arranged about the wave conductor or arranged about a Villary band as a so-called Villary detector, wherein the Villary band extends from the wave conductor in particular at a 90° angle and is connected with the wave conductor in particular mechanically fixated, e.g. welded so that the torsion impulse running in the wave conductor is transformed in the Villary band into a longitudinal wave. A longitudinal wave of this type compresses or expands the magneto-elastic element, thus the wave conductor or the Villary band in an elastic manner in the crystalline range and thus changes its permeability p. The Villary band or the wave conductor for this purpose is made from a material with a maximum change of magnetic permeability Δμr, e.g. from nickel or a nickel alloy or from other suitable materials. As a compromise between the desired properties thus also so-called constant module alloys have proven useful in which the temperature coefficient of the elasticity module and/or of the shear module can be influenced over wide temperature ranges and in particular can be kept constant. Thus, e.g. the form of a band material with intrinsic stability and a thickness of approximately 0.05 to 0.2 mm and 0.5 to 1.5 mm width are being used.
Due to
                    Δ        ⁢                                  ⁢        U            ≈                        -          N                ×                  Δϕ                      Δ            ⁢                                                  ⁢            t                                →                  Δ        ⁢                                  ⁢        U            ≈                        -          N                ×                              Δ            ⁢                                                  ⁢            B            ×            A                                              -              Δ                        ⁢                                                  ⁢            t                                =      N    ×    A    ×                  μ        ⁢                                  ⁢        0        ⁢                                  ⁢        μ        ⁢                                  ⁢        0        ×        ΔμΓ        ×        H                    Δ        ⁢                                  ⁢        t            
The following result is achieved:
      Δ    ⁢                  ⁢    U    ≈                    Δμ        ⁢                                  ⁢        r                    Δ        ⁢                                  ⁢        t              ×    K  since the values for μ0 I, N, L can be presumed to be constant.
The mechano-elastic density wave running through a magneto-elastic element e.g. the Villary band thus provides a voltage change ΔU which can be tapped at the detector coil as a usable signal.
It is apparent that the usable signal ΔU is the greater, the greater the change of the magnetic permeability Δμr.
Additionally a portion of the curve Δμr(H) plotted as an operating point or operating portion of the curve Δμr(H), thus of the magnetic permeability over the magnetic field strength is desirable in which the magnetic permeability Δμr changes in a possibly linear manner in the strongest way possible relative to its cause. Therefore it is attempted to configured the function Δμr(H) in the rising flank as steep as possible and to establish the operating portion therein in the approximately linear portion.
In the prior art a so-called bias magnet configured as a permanent magnet is used for adjusting the operating point, wherein the bias magnet is arranged physically proximal to the detector coil, e.g. parallel to the Villary band.
The operating point of the mechano-elastic detector unit besides the magnetic parameters of the bias magnet is mostly dependent from its positioning relative to the detector coil.
However, it is a disadvantage that detector coils are relatively large and not extremely reliable.
When they shall be additionally protected against interference from magnetic fields through a housing made from ferrite material they also become respectively large.
As magneto-resistive elements additionally field plates, hall sensors and XMR-sensors are known. These, however, due to their lower sensitivity and the higher background noise are less effective and therefore have not been used for this application so far.
Thus while hall sensors react to a change of magnetic induction (B), XMR-sensors, thus thin layer sensors which change their resistances as a function of magnetic flux strength and orientation react directly to a change of the magnetic field strength (H) and its direction.
Thus the term “XMR” sensor is a general term for different types of magneto-resistive (MR-sensors), namely e.g.                AMR sensors which use the anisotrope-magneto-resistive effect        GMR sensors which use the giant-magneto-resistive effect        TMR sensors which use the tunnel-magneto-resistive effect        CMR sensors which use the colossal-magneto-resistive effect        GMI sensors which use the giant-magneto impedance effect, and        MTJ sensors which use the magnetic-tunneling junction-effect.        
The disadvantage of reduced sensitivity, however, is balanced by simple producability or cost-effective availability and simple processability and simple mechanical handling of XMR sensors typically provided in the form of a microchip.